Piezoelectric oscillator system



Nov. 16, 1954 E. l.. MINNlcH 2,694,779

PIEZOELECTRIC OSCILLATOR SYSTEM Original Filed March 7, 1949 ETE- l 4Shee'ucs-Sheet l f/v VEA/ro@ 205 Q/I @Awa/J Q. 915W@ NOV 16, 1954 E. L..MINNlcH PIEZOELECTRIC OSCILLATOR SYSTEM Original Filed March '7, 1949Nov. 16, 1954 E. L. MINNICH PIEzoELEcTRIc oscILLAToR' SYSTEM 4Sheets-Sheet 3 Uriginal Filed March '7, 1949 2:1 LZ--l 44 Nov. 16, 1954E. 1 MINNICH 2,694,779

PIEzoELEcTRIc oscILLAToR SYSTEM Original Filed March 7, 1949 4Sheets-Sheet 4 r E 1E- 1 5 5 a l E @x5/won@ 52a @16M 55 56 6)' Tra- E u@MLM United States Patent O PlEZELEC'lRIC DSCILLATOR SYSTEM .Edward L.Mnnlich, Carlisle, Pa., assigner to Selectronics, inc., Carlisle, Pa., acorporation of Delaware @riginal application March 7, 1949, Serial No.79,939, now Patent No. 2,542,045, dated February 20, 1951. Divided andthis application September 23, 1949, Serial No. 117,292

'7 Claims. (Ci. 250--36) My invention relates broadly to lpiezoelectriccrystal oscillator systems and more particularly to a circuitarrangement for a piezoelectric crystal system for sustainingoscillations at selected frequencies.

This application is a division of my application Serial Number 79,939,tiled March 9, 1949, for Piezoelectric Crystal System, now Patent No.2,542,045, dated February 20, 1951.

One of the objects of my invention is to provide an improvedpiezoelectric crystal controlled electron tube oscillator circuit inwhich the mounting for the piezoelectric crystal includes meanssupplementing the piezoelectric crystal for facilitating the generationof oscillations in the oscillator circuit.

Another object of my invention is to provide an improved piezoelectriccrystal electron tube oscillator circuit in which a piezoelectriccrystal is mounted within a metallic housing which also eneloses ametallic plate capacitatively related to the said housing andelectrically connected in the circuit of the electron tube oscillatorfor facilitating the sustaining of oscillations generated by thepiezoelectric crystal.

Still another object of my invention is to provide a mounting means fora multiplicity of piezoelectric crystals within a metallic housingincluding means for electrically shielding means to the associatedmounting means in such a manner that certain electric condensersnormally required in the co-acting oscillator circuit may be omitted andeliminated, thus saving cost in production of electrical apparatusemploying such circuits.

Other and further objects of my invention reside in the circuitarrangement for a piezoelectric crystal oscillator system as set forthmore fully in the specification hereinafter following by reference tothe accompanying drawings, in which:

Figure 1 is a top plan View of one form of multiple piezoelectriccrystal holder employed in the oscillator system of my invention; Fig. 2is a longitudinal sectional view through the multiple piezoelectriccrystal holdertaken substantially on line 2 2 of Fig. 1 and Fig. 4; Fig.3 is a longitudinal sectional view through the multiple piezoelectriccrystal holder taken substantially on line 3--3 of Fig. 1; Fig. 4 is atransverse sectional view taken substantially on line 4 4 of Fig. l;Fig. 5 is a schematic wiring diagram illustrating the manner ofselecting the individual piezoelectric crystals in the multiplepiezoelectric crystal apparatus of my invention; Fig. 6 is a perspectiveView of the multiple piezoelectric crystal holder showing the parts ofthe holder in juxtaposition ready for assembly in a compact assembly;Fig. 7 is a transverse sectional view taken through the assembledpiezoelectric crystal holder shown in Fig. 6 with the parts assembled,the view being taken substantially on line 7-7 of Fig. 6; Fig. 8 is aperspective View showing the capacitor element employed within thecasing of the apparatus; Fig. 9 is a schematic view showing theoscillator circuit arrangement of my invention by which certaincondensers in the oscillator are compensated for and omitted oreliminated by use of the construction of multiple piezo crystal holderof my invention; Fig. 10 is a top plan View of a modied form of multiplepiezo crystal apparatus employing my invention in which individualinsulators are employed for insulating the piezoelectric crystals fromthe metallic housing of the piezoelectric crystal apparatus; Fig. 1l isa longitudinal sectional view taken substantially on line- 11-11 of Fig.10 and Fig. 13; Fig. 12 is a longitudinal sectional view through themultiple piezoelectric crystal apparatus taken substantially on line12-12 of Fig. 10 and Fig. 13; Fig. 13 is a transverse sectional viewtaken substantially on line 13 13 of Fig. 10; Figs. 14, l5 and 16 showmodiied arrangements for fixing the effective capacity of the capacitorthat is built into the multiple crystal holder by specially relating thecapacitor strip by differing dimensional amounts with respect to thecasing; Fig. 17 is a perspective View of the piezoelectric crystalapparatus shown in Figs. 10-13 with parts broken away to illustrate theinternal construction more clearly; Fig. 18 is a transverse sectionalview through a modified form of multiple piezoelectric crystal apparatusembodying my invention; Fig. 19 is a fragmentary longitudinal sectionalview taken substantially on line 19-19 of Fig. 18; and Fig. 20 is ahorizontal sectional view taken substantially on line Zit-26 of Fig. 18.

l have developed the apparatus of my invention to meet the requirementsof electronic apparatus employing multiple piezoelectric crystals. incertain forms of radio broadcast receivers a multiplicity ofpiezoelectric crystals are employed of the order of twenty-one or more.The problem of mounting such crystals within the allowable extremelysmall area in such manner that adequate insulation is assured for theindividual crystals is a very severe one. i have shown a multiplepiezoelectric crystal holder herein in which the overall length of thecasing constituting the crystal housing for housing eleven crystals isbut 4%6, while the overall width of the casing is but lil/fm, and theheighth is but Zs I provide a. construction of multiple crystal holderwhich assures adequate insulation for the individual piezoelectriccrystals within the casing which forms an electrical shield. A capacitormember is built into the piezoelectric crystal holder and electricallyconnected with one side of each of the piezoelectric crystals andspecially related to the electrical shield constituting the casing. Thisarrangement when connected with the electron tube oscillator circuit ofmy invention completes a capacitative feed-back path for the oscillationcircuit, thereby enabling the feed-back condenser normally required inthe oscillation circuit to be omitted and eliminated. This reducesexpense in manufacture and production of radio broadcast receivers ofthe multiple piezoelectric crystal type which in a mass productionprogram may involve very sizeable amounts. The appa: ratus of myinvention involves novel mounting means for the individual piezoelectriccrystals by which adequate support of each of the piezoelectric crystalswithin the shielded housing is assured in a very practical manner. I mayemploy piezoelectric crystals of various cuts and types as typiiied forexample by the circular crystal of Fig. 4 or the rectangle crystals ofFigs. 6, 7, 13 and 17, 18, 19 and 20.

Referring to the drawings in detail, reference character 1 designatesthe metallic shielding container or tank having an open top terminatingin a peripheral edge 2 and having a closed metallic bottom 3. Extensionlug mem bers 4 and 5 are secured to opposite ends of the external faceof the closed bottom 3 of the shielding container 1. The lugs 4 and Seach contain apertures 4a and 5a that permit the passage of securingscrews 6 and 7 which extend through the washers Si and 9 and enter thesupporting surface such as the chassis of associated electronicapparatus, which may be a radio receiver and the like.

A panel formed from insulation material 10 having a peripheral ange 10athereon enters the open end 2 of the tank 1 and is secured with respectto the tank 1 by appropriate securing screws 11. l mount a multiplicityof terminal posts 12 corresponding in number to the number ofpiezoelectric crystals supported in the holder on the panel ofinsulation material 1t? in a row extend ing longitudinally of the holderas shown in Figs. 1 and 2.

In positions transversely opposite each of the end terminal posts in therow of terminal posts 12 I provide terminal posts 14 insulatingly andspecially related thereto. Terminal posts 14 serve as supports andelectrical connectors for the capacitor strip 15 which extendslongitudinally of the casing 1. The strip 15 is formed from metal stockapproximately .040 to .050 thick and 1A" wide. in the dimensionalexample heretofore given this strip is approximately 4 in length. In theform of my invention shown in Figs. 1 8 the capacitor strip 15 is in theform of an angle having ilanges extending in planes substantially normalto each other with one flange thereof, approximately 1A wide, extendingin spacial capacitive relation with the interior side wall of metalliccontainer 1. This forms a condenser in the circuit path of each of thepiezoelectric crystals. Small studs 16 are welded or otherwise securedto strip and project therefrom in an upright position serving assupports for springs 17. In the dimensional example heretofore referredto these studs are spaced approximately 3/8 apart which permits mountingof a plurality of spaced crystals within the container 1 without mutualcontact. Springs 1'7 conneet with one of the ends of the severalpiezoelectric crystals 18 shown in Fig. 4 and establish electrical andmechanical connection with electrode 19 thereon. The other ends of thepiezoelectric crystals 18 are supported in springs projecting fromterminal posts 12 and establishing electrical and mechanical connectionwith electrodes 21 on the opposite faces of the crystals.

The terminal posts 12 and 14 have annular anges thereon that areembedded in the insulation material of panel 10 for solidly anchoringthe posts in the panel. The posts 12 and 14 project through the materialof panel 10 and provide external connection means to the electronicoscillator system outside of the casing 1. The terminal posts 14adjacent opposite ends of casing 1 serve as both electrical connectingmeans and mechanical supports for the strip 15.

The springs 17' and 2t? which serve as supporting members for theindividual piezoelectric crystals 18 comprise coils of spring wire 17aand 20a which respectively embrace the inwardly directed studs 16 andthe inwardly directed terminal posts 12. The other ends of springs 17and 20 terminate in looped or coiled ends 17h and Ztlb disposed in thesame plane in spaced positions within the container 1 to grip theelectrodes of the piezoelectric crystal 13 therebetween.

Each piezoelectric crystal 13 has the opposite conductive electrodes 19and 21 thereof which are secured to opposite faces of the piezoelectriccrystal intimately connected with the spring wire loops or coils 17b and2017 by low melting point metal which assures a rigid mechanical supportbetween the spring wires 17 and 20 and the piezoelectric crystal 1S anda highly conductive electrical connection with the electrodes 19 and 21of the crystal 1S. This connection is so sturdy and reliable that venthough the multiple piezoelectric crystal holder is subjected 'to verysubstantial vibration or in fact, mechanical shock, the crystals areretained in the grip of the spring wire coils or loops 17 b and 2Gb. Byreason of the structural section of the strip 15 such vibration or shockdoes not appreciably displace the strip or impair the establishedcapacitative relation thereof to the wall of the metallic container 1.

In Fig. 5 l have illustrated the circuit arrangements for thepiezoelectric crystal elements from which it will be noted that onelongitudinal row of terminals is constituted by capacitor strip 15,capacitatively related to the metallic wall of container 1 which isgrounded at 22. The other row of terminals is constituted by spacedterminal posts 12 which individually connect with the opposite sides ofeach of the piezoelectric crystal elements. The several piezoelectriccrystal elements connect to contacts arranged compactly in the arc of acircle as represented at 23, over which the switch arm 24 operates toselect a particular piezoelectric crystal from the group ofpiezoelectric crystals 18. In Fig. 5 I have schematically shown onlyfive piezoelectric crystals but it will be understood that any multiplenumber of such elements may be employed.

in Fig. 6 l have shown the shielded container 1, removed from the panel10 of insulation material and disposed in juxtaposition with respectthereto, and showing in perspective View the piezoelectric crystalelements which are normally housed within the container. Thepiezoelectric crystal elements in this arrangement are rectangular, asshown at 18a, instead of circular as shown in Fig. 4 at 18, but allother details are arranged as cxplained in connection with Figs. 1-5.The posts 12 in this arrangement are illustrated as having solder lugs25 connected to the externally projecting stems of the terminal postsand extending in spaced relation beyond the periphery of the panel 10for permitting individual electrical connection to be readily madethereto. The electrodes on the surfaces of the piezoelectric crystalsare arranged m a manner explained in connection with Fig. 4 but byreason of the rectangular shape of the crystals the electrodes areelongated as represented at 19a and 21u of Figs. 6 and 7.

Fig. 7 illustrates on a somewhat larger scale the manner m which therectangular piezoelectric crystals are spaclally disposed withincontainer 1 while capacitative coupling is established between strip 15and the metallic wall of container 1.

in Fig. 9 I have represented the electrical circuit of the electron tubeoscillator which is associated with the multiple piezoelectric crystalswithin the container 1. The metallic Wall of the metallic container ortank 1 forms one capacity area of an electrical condenser in co-actionwith the capacitor strip 15. Electron tube oscillator 26 isschematically shown as including cathode 26a, control grid 26h and ananode 26C. The control grid 26h is connected in the input circuit of theoscillator through the impedance 2'7 to ground 22. Cathode 26a isconnected with the output circuit of the oscillator including radiofrequency choke coil 29 and high potential source indicated at 30, andground connection 22. The piezoelectric crystal elements represented at18, 18', 1S", etc. have the electrodes 19, 19', 19", etc., thereof,respectively connected to capacitor strip 15.

The electrical capacity between electrodes 19, 19', 19 and the metalliccontainer 1 is established by the spacial relation of metallic strip 15and the metallic wall of the container 1. The electrical capacity thusprovided constitutes a feed-back path for oscillations between the inputand output circuits of the oscillator 26. This capacity compensates forand takes the place of an external condenser which must normally beincluded in the input circuit of the oscillator in shunt with impedance27 in the input circuit. Accordingly, by spatially relating capacitorstrip 15 to metallic container 1 internally of the container, theessential feed-back capacity may be provided between the input or outputcircuits of the oscillator without the necessity of installing theseparate condenser normally required. This results in a very appreciablesaving in a manufacturing and production program where many thousands ofsuch condensers would be required.

in Figs. 10-20 I have shown a modified form of multiple crystal holderwhich may be used in the circuit of my invention in which the metallichousing 31 is substituted for the shielding container or tank 1. Themetallic housing 31 has a peripheral bead 32 adjacent the extremity ofthe edge thereof which provides a peripheral seat for the invertedmetallic pan member 33.

The inverted metallic pan 33 is peripherally supported by the peripheralseat 32 when the pan 33 is frictionally forced into the housing 31. Thepan 33 forms a support for the multiplicity of piezoelectric crystalelements. In this arrangement one row of piezoelectric crystal elementsupports represented by the resilient Wire-like spring elements 34 areattached by coils 34a to studs 35 depending from capacitor strip 36 andterminate in loops 34h which embrace the ends of piezoelectric crystalelements 18h for establishing electrical connection with the electrodesurface 19h and 2117 as represented in Fig. 13. The capacitor strip 36is insulatingly and spacially mounted with respect to pan 33 by means ofterminal posts 44 that pass though bushings 37 of insulation materialsupported in sleeve-like bushings 3S extending through the pan 33. Acommon terminal connection for the container 31 projects from the rearof pan 33 as shown at 39. The co-acting opposite terminals for each ofthe multiple number of piezoelectric crystal elements are insulated fromthe metallic pan 33.

' Terminal posts 4t) individual to each of the resilient wire-likemembers 41 are gripped by the coiled ends 41a of the resilient wire-likemembers 40 as shown in Fig. 13. T he terminal posts 4G pass throughbushings formed by insulation material 42 carried in sleeve-like members43.

The sleeve-like members 43 each have annular flanges 43a thereon and areseated in a longitudinally extending row of apertures provided in pan33. The insulation material 42 is vitreous or is formed from compositeinsulation material and substantially insulates terminal post 4t) fromcontact with metallic sleeve 43 or metallic pan 33. The terminal posts40 projects beyond the peripheral'limits of vpan 33 andenableielectr'ical connections to be made thereto for'completingcircuits to the individual piezoelectric crystal elements.

The opposite extremities orv the metallic housing 31 or the pan 33 maybe rolled over to provide a peripheral interlocking joint, or spacedlugs 45 may be provided on housing 31 and may be rolled over theperipheral edge of pan 33 to secure pan 33 in position againstperipheral seat 32 and within housing 3l.

The same method of mounting the metallic housing 31 is provided as isemployed in the arrangement of Figs. 1-8; that is, lugs i and 5 areperforated at 4a and 5a for the passage of securing screws.

The electrical capacity referred to in Fig. 9 is inherently provided inthe multiple piezoelectric crystal holder of Figs. 10-13, eliminatingand replacing feed-back condenser normally required in the oscillatorcircuit. In order that different values of capacity may be providedwithin the crystal holder l may selectively lix the length of insulationbushings 37 as represented in Figs. 14, ,15 and 16 to fix the spacialrelation of capacitor strip 36 and the metallic pan 33. For example, theshort bushing 37 in Fig. 14 provides a relatively high capacity value;the bushing 37 of greater length shown in Fig. provides a capacity oflower value, while the bushing 37 of the longest length as shown in Fig.16 provides an effective capacity within container 31 of the smallestvalue.

The manner of supporting the piezoelectric crystals in the form of myinvention illustrated in Figs. 10-13 is the same as that described inconnection with the form of my invention shown in Figs. l-8, and I haveapplied similar reference characters to corresponding parts of thecrystal and the electrodes thereof by adding the subscript a to thecrystal and the electrodes thereof.

ln Figs. 18-20 l have illustrated a further modified form of multiplecrystal holder in which a metallic housing 46 is provided, closed on allsides except one which is closed by a metallic pan 47 having aperipheral edge 48 operative to engage the edge of housing 46. The pan47 is apertured in transverse aligned positions at 49 and 50 withoutstruck sleeves 49a and 56a co-extensive with the pan 47. rfhe sleeves49a and 50a are filled with insulation material 51 and 52 formingsupports for 'the pin members 53 and 4.

The pin members 53 and 54 each carry spring strips on the ends thereofas represented at 55 and 56. The spring strips S5 and 56 are each foldedupon themselves to form spring clamping jaws 57 and 58. These springclamping jaws grip and engage opposite sides of the piezoelectriccrystal which in this instance is represented by reference character 59with opposite electrodes 60 and 61 thereon. The electrodes 60 and 61extend from the center of crystal 59 to opposite corners serving as ametallic coating which is gripped by the spring clamping jaws 57 and 58.Solder or low melting point metal is applied between the metallicelectrode 60 and spring jaw 57 and between metallic electrode 51 andspring jaw 58 for electrically and mechanically maintaining thepiezoelectric elements in position in the multiple piezoelectric crystalholder. This clamping method is comparable to the clamping methodemploying the spring coils or loops of Figs. 1-13, in that thepiezoelectric crystal elements are maintained in position and cannot beaccidentally or readily shaken or displaced from the securing means.

The capacitor strip 62 is mounted on pin members 5.3 spaced from pan 47by the insulation supports 51 in capacitative relation to the undersideof pan 47. An electrical circuit grounded to the metallic housing 46inherently places the electrical capacity existent between the groundand one of the electrodes of the piezoelectric crystals in shunt withthe input circuit of the o scill'ator in the manner explained inconnection with the circuit of Fi 9.

gNith the advent of piezoelectric crystal apparatus functioning on manyclosely adjacent frequencies in multiple signaling channels, theimportance of the multiple piezoelectric crystal apparatus of myinvention will be appreciated. The large number of individualpiezoelectric crystal elements of different frequency characteristicsmay be mounted within an extremely small space on racks or adjacent anapparatus chassis for efficiently providing for the selective operationof numerous electrical circuits on closely adjacent frequencies. a

One of the advantages arising out of .the application of the multiplecrystal holder of my invention results from the fact that it is diilcultand costly to finish a crystal to a precisefrequency, `andthe wider thefinishing tolerance', theslower the production cost. Crystals whichcover the broadcast band maybe ground to a relatively wide finishfrequency tolerance by controlling the amount of plating on the surfacesof the crystals, and then grouping or classifying these crystals as tothe number of cycles to which they may be high or low withrespect to theprecise frequency or desired frequency. The crystals may be classied inat least three groupsas being high, medium (or on), and low, andmayinclude two other classifications, as extremely high and extremely lowfrequency tolerance crystals.

An oscillator circuit does have some influence on the frequency of thecrystal, and if the grid to ground capacity of a Pierce oscillator, forexample, is varied the frequency is changed appreciably and vmay bringthe crystals into tolerance, providing they are finished at somefrequency not too 'far removed from the desired frequency. lf thecrystal frequency is higher than the desired frequency the capacitywould be increased across the grid to ground. Conversely, if the crystalfrequency is lower than the desired frequency, the capacity is reduced.

From the foregoing it will be understood that the holder may be made inthree to five steps with respect to the value of this built-incapacitor, which is connected between the grid of the oscillator and theground. The crystals noted as high or extremely high are put in holderswith the maximum value of inbuilt capacity. The crystals classiiied aslow or extremely low are put in a holder with the minimum capacity, andthose crystals classified as medium or on would go into a holder with anaverage or medium value of capacity. As hereinbefore stated this makesthe finishing operation considerably more simple, thereby reducingmanufacturing costs. This isa substantial advantage to the crystalmanufacturer and can be passed on to the customer in lower price of theunits.

While have describedmy invention in certainvof its preferred embodimentsI realize that modifications may be made in the arrangement of parts7the circuit details, and elements of the apparatus and I desire that itbe understood that no limitations upon my invention are intended otherthan may be imposed by the scope of the appended claims.

What i claim as new and desire to secure by Letters Patent of the UnitedStates is as follows:

l. A piezoelectric crystal system comprising in combination, an electrontube having input and output circuits, a piezoelectric crystal systemincluding a piezoelectric plate, a metallic envelope extending from abase of insulation material and enclosing said piezoelectric crystalplate, a capacitor strip mounted inside said envelope in spatialinsulated relation to the interior surface of one of the metallic wallsthereof and a plane substantially normal to the plane of saidpiezoelectric plate and electrically connected with one side of saidpiezoelectric crystal plate, and electrical connections extendingthrough said base of insulation material between the input and outputcircuits of said electron tube and one side of said envelope, theelectrical capacity existent between said envelope and said capacitorstrip providing an electrostatic feed-back path between said circuits.

.2. A piezoelectric crystal system comprising in combination an electrontube having input and output circuits, a piezoelectric crystal elementelectrically connected between said input and output circuits, ametallic envelope extending from a base structure of insulation materialand surrounding said piezoelectric crystal element, circuit connectionsextending through said base structure, a capacitor element disposedinside said envelope in insulated spatial relation to one interior wallof said metallic envelope and extending in a plane substantially normalto the plane of said piezoelectric crystal element and electricallyconnected with one side of said piezoelectric crystal element forestablishing by said spacial relation a feed-back path between saidinput and output circuits for sustaining electrical oscillations in saidcircuits under control of said piezoelectric crystal element.

3. .A piezoelectric crystal system comprising in combination an electrontube having input and output circuits, .a piezoelectric crystalelectrically connected with said circuits, a metallic envelope extendingfrom a base structure of insulation material and surrounding saidpiezoelectric crystal, circuit connections extending through said basestructure, and a metallic strip disposed inside said envelope" betweenone edge of said crystal and one of the walls of said envelope, aconnection between said strip and one side of said piezoelectriccrystal, said strip being insulatingly and capacitatively related to oneinterior wall of said envelope for establishing a feed-back path foroscillatory currents between said circuits.

4. A piezoelectric crystal system comprising in combination an electrontube having input and output circuits, a piezoelectric crystalelectrically connected with said circuits, a metallic envelope extendingfrom a base structure of insulation material and surrounding saidpiezoelectric crystal, circuit connections extending through said basestructure, and means disposed inside said envelope intermediate one edgeof said crystal and one interior Wall of said envelope and supplementingsaid piezoelectric crystal for establishing a feed-back path foroscillatory current between said circuits.

5. A piezoelectric crystal system comprising in combination an electrontube having input and output circuits, a piezoelectric crystalelectrically connected with said circuits, a metallic envelope extendingfrom a base structure of insulation material and surrounding saidpiezoelectric crystal, circuit connections extending through said basestructure, and means disposed between one edge of said crystal and theinterior wall of said envelope and capacitatively related to saidinterior Wall and electrically connected with said piezoelectric crystalfor establishing a feed-back path for oscillatory currents between saidlcircuits.

bination an electron tube including at least a cathode, a

grid and a plate and having input and output circuits, a piezoelectriccrystal, a supporting base of insulating material, supports extendingthrough said supporting base -and providing exterior terminals, meansfor mounting said piezoelectric crystal on said supports, a capacitorstrip constituted by a metallic angle member mounted on one of saidsupports, a metallic housing detachably connected with said supportingbase and enclosing said piezoelectric crystal and said metallic anglemember, said metallic angle member including anges disposed in planes extending normal to each other with one of said flanges extending inparallel relation to the interior of said base and the other of saidflanges imparting rigidity to the aforementioned flange and extending inspaced relation to one interior side wall of said metallic envelope,connections between said piezoelectric crystal and the grid and plateelectrodes of said electron tube, and electrical connections between theinput and output circuits of said electron tube and one side of saidenvelope, the electrical capacity existent between said envelope andsaid capacitor strip providing an electrostatic feed-back path betweensaid circuits.

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date 1,933,979 Hentschel Nov. 7, 1933 1,994,228 Osnos Mar. 12, 19352,260,707 Fair Oct. 28, 1941

